Make the sdt module public

It was pointed out that users can't actually access Signature or anything
else in this module, because it's private. I think this is just an oversight
so we'll make it public.

acpi/src/lib.rs

@@ -51,7 +51,7 @@ mod hpet;
 mod madt;
 mod mcfg;
 pub mod platform;
-mod sdt;
+pub mod sdt;
 
 pub use crate::{
     fadt::PowerProfile,